In recent years, from the viewpoint of global environmental protection, improvements in automotive fuel efficiency have been directed in the whole automotive industry for the purpose of regulating CO2 emissions. Automotive weight reduction by the gauge reduction of parts used is most effective in improving the fuel efficiency of automobiles. Therefore, in recent years, the consumption of high-strength steel sheets for automotive parts has been increasing.
On the other hand, the formability of steel sheets tends to deteriorate with an increase in strength. Therefore, steel sheet having excellent formability in addition to high strength are demanded. A steel sheet which is short of stretch flangeability cannot be applied to underbody parts or the like needing stretch flangeability. The development of a steel sheet having high strength and stretch flangeability is essential to lighten automotive parts and the like. Various techniques focused on stretch flangeability have been proposed for high-strength cold-rolled steel sheets and hot-dip coated steel sheets.
For example, Patent Literature 1 describes that a high-strength galvanized steel sheet excellent in formability is obtained. The high-strength galvanized steel sheet has a composition containing C: 0.05% to 0.3%, Si: more than 0.6% to 2.0%, and Mn: 0.50% to 3.50% on a mass basis and a microstructure containing a ferrite phase, a tempered martensite phase, a tempered bainite phase, and a bainite phase, wherein the area fraction of the ferrite phase is 20% or more, the sum of the area fractions of the tempered martensite, tempered bainite, and bainite phases is 10% or more, and the sum of the area fractions of the ferrite, tempered martensite, tempered bainite, and bainite phases is 90% or more.
Patent Literature 2 describes that a high-strength galvanized steel sheet, excellent in workability, having a TS of 1,200 MPa or more and a hole expansion ratio of 50% or more is obtained. The high-strength galvanized steel sheet has a composition containing C: 0.05% to 0.5%, Si: 0.01% to 2.5%, and Mn: 0.5% to 3.5% on a mass basis and a microstructure containing a ferrite phase, a martensite phase, a tempered martensite phase and a retained austenite phase, wherein the area fraction of the ferrite phase is 0% to 10%, the area fraction of the martensite phase is 0% to 10%, the area fraction of the tempered martensite phase is 65% to 95% and the ratio of the retained austenite phase determined by X-ray diffractometry is 5% to 20%.